1. Field of the Invention
The present invention relates to a direct-acting device such as linear guide and ball screw to be incorporated in industrial machines, etc. and more particularly to a technique for reducing noise.
The present invention also relates to a rolling device for use in conveyance system and positioning device in semiconductor producing machine and sputtering device, plasma CVD device and ion planting device in liquid crystal, semiconductor and solar cell producing apparatus and more particularly to a low-dust generating and high durability rolling device which can be preferably used in vacuum atmosphere.
The present invention also relates to a rolling device for use in conveyance system and positioning device in semiconductor producing machine and sputtering device, plasma CVD device and ion planting device in liquid crystal, semiconductor and solar cell producing apparatus and more particularly to a low-dust generating, low-outgassing and high durability rolling device which can be preferably used in vacuum atmosphere or clean room.
The present invention also relates to a rolling device such as linear guide and ball screw to be incorporated in industrial machines and a separator for rolling device and more particularly to an effective technique which can be used in a clean atmosphere such as clean room, semiconductor producing machine, liquid crystal panel producing machine and hard disc producing machine.
2. Description of the Related Art
As shown in FIG. 1 for example, a linear guide device 110 comprises a guide rail 101 having a rolling groove 103 provided on the outer surface thereof, and a slider 102 disposed across the guide rail 101. The slider 102 is partly opened on the surface thereof opposed to the rolling groove 103 of the guide rail 101 to form a ball circulating path 104 having a racing track-shaped section with the rolling groove 103 of the guide rail 101. Into the ball circulating path 104 are received rollably a large number of balls B.
Alternatively, as shown in FIG. 2, a ball screw device 120 comprises a ball nut 112 provided surrounding a thread 111, and a plurality of balls B rollably disposed in the space defined by a thread groove 112a formed spirally on the inner surface of the ball nut 112 and a thread groove 111a formed spirally on the outer surface of the thread 111 opposed to the thread groove 112a. The ball nut 112 has a ball tube 113 having an external shape of U mounted with its both ends extending to the thread groove 111a of the thread 111. The balls B make a repeated circulation. In some detail, the balls B run around the thread 111 plural times inside the ball nut 112. The balls B are then caught by one end of the ball tube 113 from which they then pass through the ball circulating path 118. The balls B are then returned to the thread groove 111a of the thread 111 from the other end of the ball tube 113.
Such a linear guide device 110 or ball screw 120 normally has a separator interposed between the balls B to eliminate noise of collision of balls during driving. For example, FIG. 3 is an enlarge view illustrating the interior of the ball circulating path 104 of the linear guide device 110. A train of balls B with a separator 130 interposed therebetween is formed. The separator 130 has a concave surface 131 having an arc section formed on both sides thereof corresponding to the outer surface of the ball B. The ball B is rollably retained by the concave surface 131 when circulating through the ball circulating path 104.
The aforementioned separator 130 is a product of a resin composition containing a proper reinforcing material. As the resin composition there has heretofore been normally used one comprising as a base resin a polyamide resin such as nylon 66. However, a polyamide resin itself has poor sliding properties. While there is a sufficient amount of a grease, the liner motion device can operate stably with the lubricating action of the grease. However, when the grease flows out of the liner motion device with water to an extent such that only a slight amount of the grease is left, there occurs a sudden deterioration of the operating characteristics of the liner motion device, possibly increasing the noise level due to running.
A ball screw, which is one of rolling devices, comprises a plurality of balls (rolling elements) disposed in a spiral space (hereinafter referred to as “ball rolling space”) defined by a thread groove (track) on the thread (inner member) and a thread groove (track) on the ball screw nut (outer member). In this arrangement, power is transmitted between the thread and the ball screw nut when the balls are rolled and circulated. Since the balls are rolled with slippage developed by the resultant of force in the rotary direction and force in the axial direction, the contact area of the ball with the both thread grooves (both tracks) defining the ball rolling space undergoes rolling friction and sliding friction at the same time.
Thus, a lubricant such as grease and solid lubricant is used in the interior of the ball screw. This lubricant lessens the coefficient of friction between the thread and ball screw nut and the balls, enhancing the durability (life) of the ball screw. However, when as the lubricant for ball screw to be used in vacuum atmosphere there is used a grease, the evaporation of the oil content of the grease or the scattering of the grease itself can cause defects such as deterioration of lubricating properties and pollution of the working atmosphere.
In order to eliminate these defects, it has been practiced to coat at least any one of the raceway surfaces such as thread groove on the thread and thread groove on the ball screw nut and the surface of the balls with a solid lubricant such soft metal (e.g., gold, silver, lead), carbon and molybdenum disulfide in film form to provide these surfaces with lubricating properties instead of using grease. Further, Japanese Patent Laid-Open No. 08-226516 discloses a ball screw comprising a lubricant film made of a fluorine-containing polymer having a functional group which can be used in vacuum atmosphere, clean atmosphere and corrosive atmosphere.
Moreover, Japanese Patent Laid-Open No. 2001-193814 discloses a liner motion device comprising a retaining peace (spacer) interposed between balls.
However, the aforementioned coat layer made of such a solid lubricant or the like is disadvantageous in that the load developed by the rolling of balls causes the coat layer to be gradually peeled off and fall off, resulting in the generation of dust.
Further, when the coat layer is peeled off or falls off as mentioned above, the site at which the rolls are rolled and make sliding contact with the thread and the ball screw nut is less lubricated. As a result, the metals come in contact with each other, causing cohesion or accelerating abrasion at the rolling and sliding site and hence deteriorating the life of the ball screw.
Moreover, the aforementioned lubricant film made of a fluorine-containing polymer having a functional group is fluidic and thus can be difficultly peeled off or fall off unlike the coat layer made of solid lubricant or the like. Thus, the ball screw having such a lubricant film is less subject to abrasion as compared with the ball screw having a coat layer made of solid lubricant or the like. In general, however, when the balls collide with each other as they roll, the adjacent balls rotate in opposite direction (reverse operation). Thus, the sliding speed of balls relative to each other is twice that of a single ball, causing a great frictional force. Accordingly, the contact of the balls with each other has a great effect on the durability of the ball screw, possibly deteriorating the life of the ball screw.
Rolling devices such as rolling bearing, linear guide device, ball spline device, ball screw device, cross roller bearing and cam follower are used in various arts.
In general, rolling devices such as rolling bearing and liner motion device are coated with a lubricating oil such as mineral oil and poly α-olefin oil or a lubricant such as grease or have such a lubricating oil or lubricant filled therein to prevent abrasion of rolling element with its mating members.
However, the system involving lubrication of rolling device with a lubricating oil or grease is disadvantageous in that when the device is operated, the lubricating oil or grease is scatted to the exterior thereof. Further, when the device is operated at high temperatures, at a high speed or in vacuum atmosphere, the lubricating oil or grease vaporizes to produce a gas, contaminating the external atmosphere. Therefore, in a use requiring a clean atmosphere such as in clean room, semiconductor producing device, liquid crystal panel producing device and food processor or in vacuum atmosphere, rolling devices which are lubricated with an ordinary lubricating oil or grease cannot be used.
Therefore, in a use requiring a clean atmosphere or in vacuum atmosphere, a rolling device comprising a retainer made of a self-lubricating composite material mixed with a solid lubricant or the like, a solid-lubricating rolling device having a film made of a solid lubricant formed on the surface of various elements constituting the rolling device (e.g., guiding surface of moving member, surface of rolling element) or a rolling device which is lubricated with a fluorine-based grease having a low vapor pressure and a small consistency is used.
These rolling devices can be used without any problems under ordinary working conditions. However, when used at high temperatures, at a high speed or in vacuo, these rolling devices cause the lubricant to be scattered or evaporated as a gas to the exterior thereof, contaminating the external atmosphere thereof. Therefore, in devices requiring a clean atmosphere such as device for use in a clean room, e.g., semiconductor producing device, liquid crystal panel producing device and hard disc producing device, vacuum devices having a rolling device exposed to vacuum such as vacuum pump, and devices which are used at high temperatures, a fluorine-based grease has been often used as a lubricant for rolling device.
A fluorine-based grease is a mixture of a base oil made of perfluoropolyether (PFPE) and a thickening agent made of polytetrafluoroethylene resin (PTFE) and thus normally has an extremely low volatility and hence is scattered or evaporated to the exterior of the rolling device in a relatively small amount. Accordingly, the fluorine-based grease can relatively difficultly contaminate the external atmosphere of the rolling device.
However, PFPE contained in the fluorine-based grease normally has a high viscosity. In other words, when such a fluorine-based grease is used as a lubricant for the rolling device, the resulting resistance to stirring is increased to raise the torque excessively, increasing the heat generation and hence expanding the members constituting the rolling device, e.g., thread, nut and rolling elements, if the rolling device is a ball screw device. As a result, the clearance between the various members is decreased, imposing excess load on the contact area of the thread and nut with the rolling elements. In particular, a ball screw device is much subject to working slippage and thus undergoes abnormal abrasion or seizing, making the rolling device unusable in a short period of time and hence making it necessary to replace the rolling device frequently to disadvantage.
When there is an increased error in mounting in a linear guide device, the contact face pressure at the rolling surface increases, making the linear guide device more subject to the aforementioned problems.
Further, the ball screw device and linear guide device make stroke movement and thus is disadvantageous in that the lubricant can easily be removed from the track surface and exhausted. In particular, the fluorine-based grease contains as a base oil PFPE, which has a small surface energy, and thus can difficultly wet the surface of the matrix, easily causing the grease to be exhausted.
A rolling bearing, particularly when used at a high speed, undergoes a raised resistance to stirring with respect to lubricants containing a base oil having a high dynamic viscosity such as fluorine-based grease. The resulting heat generation causes the internal clearance in the bearing to be reduced, making the bearing more subject to abnormal abrasion or seizing.
Further, a rolling device having a self-lubricating retainer incorporated therein causes the retainer and the rolling elements to make sliding movement relative to each other during operation. As a result, a lubricant is transferred from the retainer to the guiding surface of the moving members and the surface of the rolling elements to form a thin solid lubricant film thereon, lubricating the rolling device.
On the other hand, in a rolling device comprising a solid lubricant film, a film made of solid lubricant which has been previously formed on the guiding surface of the moving members of the rolling device or the surface of the rolling elements undergoes shearing, cleavage, etc. with the operation of the device, preventing the surfaces of the matrix from making direct contact with each other and hence lubricating the device.
As such a solid lubricant film there is used a soft metal such as gold, silver and lead, lamellar compound such as molybdenum disulfide, tungsten disulfide and graphite or fluororesin such as polytetrafluoroethylene.
However, the aforementioned soft metal or lamellar compound generates a relatively great amount of dust and thus cannot be used in a use requiring a highly clean atmosphere.
Further, a solid lubricant has different lubricating properties with different working atmospheres. For example, molybdenum disulfide can accomplish its effect in vacuo. Graphite can accomplish its effect in atmosphere. However, the semiconductor producing machine and liquid crystal panel producing machine make repeated reciprocation between atmosphere and vacuum to convey the product from one step to another, making it necessary to use a lubricant which exhibits lubricating properties both in vacuo and atmosphere. In this working atmosphere, a fluororesin is often used as a lubricant film.
In recent years, various electronic elements such as semiconductor element have attained high integration and fineness more and more. With this trend, minute particles attached to the surface of semiconductor element, etc. during production procedure have more effects on the performance, reliability and yield of the product. Therefore, rolling devices for use in places requiring a clean atmosphere as in semiconductor producing machine have been keenly required more and more to cause little particles and gases to be scattered to the exterior of the device.
In the art of liquid crystal panel, too, the same requirements as mentioned above must be met to increase the number of pixels or improve the picture quality.
However, all the aforementioned solid-lubricated rolling devices cause abrasion particles to be generated from the sliding site during the sliding of the retainer relative to the film, etc. It is unavoidable that the abrasion particles thus generated can fall off the members and then be scattered to the exterior of the device.
Further, the rolling device which is lubricated with a fluorine-based grease undergoes shearing of grease during the operation thereof. The grease thus sheared is then scattered to the exterior of the device. In recent years, the rolling devices for use in semiconductor producing machine or liquid crystal panel producing machine may be used in ultrahigh vacuum. The fluorine-based grease has no life problems but leaves something to be desired in outgassing resistance.
In the art of rolling device such as ball screw, a means for supplying a lubricant such as grease into the track surface of the nut (outer member) and the thread (inner member) on which the rolling elements are rolled to lessen rolling friction and hence enhance the durability (life) of the rolling device has been proposed.
However, there is a problem that when such a rolling device is used in vacuo in semiconductor producing machine, liquid crystal panel producing machine, etc., the vapor of the lubricant causes pollution in its working atmosphere.
Therefore, as the lubricant to be used in vacuo there has been used a lubricating oil having a low vapor pressure, a solid lubricant or the like. Examples of the solid lubricant employable herein include lamellar materials such as molybdenum disulfide, soft metal-based solid lubricants such as gold, silver and lead, and polymer-based solid lubricants such as polytetrafluoroethylene (PTFE).
However, the aforementioned electrically-conductive soft solid lubricants are disadvantageous in that they can produce an abrasion powder (particle) which are then attached to semiconductor pattern, causing short circuiting, when used in a rolling device operating in vacuo.
Further, the solid lubricant such as molybdenum disulfide and PTFE is disadvantageous in that it can difficultly be transferred to the sliding members and exhibits a deteriorated abrasion resistance that gives an insufficient durability.
The present applicant proposes a rolling device comprising a dense nitride layer formed thereon and having its track surface coated with a lubricant made of a fluorine-containing polymer having a functional group and a perfluoropolyether (PFPE) in Japanese Patent Laid-Open No. 2001-248708.
The present applicant also proposes a rolling device having its track surface coated with a lubricant comprising as a main component an alkylated cyclopentane having a vapor pressure of not higher than 10−11 Torr at 20° C. in Japanese Patent Laid-Open No. 2001-72987.
A rolling device having its track surface coated with such a lubricant generates a reduced amount of dust or causes little outgassing and exhibits a prolonged life as compared with rolling devices comprising a solid lubricant.
In a rolling device such as ball screw, a means has been proposed involving the disposition of a separator (retention peace) interposed between adjacent balls for the purpose of realizing noise reduction and life prolongation and the arrangement such that the separator moves with the rolling of the balls to prevent competition of the adjacent balls.
This separator has a columnar shape and a concave surface formed on both bottom surfaces thereof for retaining the ball. Further, the separator is formed by an unreinforced product of a resin composition or a product of such a resin composition containing a proper reinforcing material. As such a material constituting the resin composition there may be used a polyamide resin such as nylon 66.
In recent years, with the trend of enhancement of degree of integration and fineness of various devices such as semiconductor element, there has been an apprehension that fine particles and gases can be attached to the surface of semiconductor elements during production to have adverse effects on product performance, reliability and yield. It has thus been keenly desired to not only realize the prolongation of the life of the rolling device but also minimize the amount of particles or gases to be scattered or evaporated from the rolling device to the exterior thereof.
The aforementioned rolling device having its track surface coated with a lubricant generates a reduced amount of dust or causes little outgassing. However, since the rolling device is lubricated by a thin lubricant film, the life of the rolling device expires sometime when the lubricant film is lost. Therefore, in order to render the rolling device maintenance-free or prolong the maintenance-free life of the rolling device, some problems must be solved.
On the other hand, the rolling device having a separator incorporated therein is disadvantageous in that the polyamide resin which is used as a constituent of the separator absorbs water to release gas.
The polyamide resin which is used as a constituent of the separator is also disadvantageous in that when the rolling device is operated unlubricated or with a slight amount of a lubricant, the resin undergoes remarkable abrasion, causing the life of the rolling device to expire in a short period of time and the generation of abrasion powder that pollutes the external atmosphere of the rolling device.